As Washington watched the beginning of a government shutdown Tuesday, people whose businesses depend on the government waited, too — with even less idea of what the near future may hold than many government workers.

From defense contractors to Realtors whose clients are applying for government-insured mortgages, millions of Americans' employers do business with the government. The federal offices shut down non-essential services at midnight Monday, but companies aren't prepared to say much about how they might weather a shutdown.

The biggest reason for the uncertainty is that businesses are still waiting for details of what contracts will be honored during the shutdown and which ones will be suspended until Congress appropriates new funding, said Bruce Herskovics, a senior bond analyst for Moody's Investors Service.

"One of the hardest things about the shutdown is uncertainty,'' Herskovics said. "Not just about which contracts are affected but also about how long it will last.''

Economists largely agree that the impact of a short shutdown will be fairly small, but will widen and deepen if the impasse persists.

A three-week shutdown would reduce fourth-quarter economic growth by 0.5 percentage points, or almost a third of the growth rate of the last 12 months, IHS Global Insight said. A shutdown that lasts a few days would cut growth by about 0.2 points, IHS chief U.S. economist Doug Handler said.

The government has not yet given an official estimate of worker furloughs, but Handler estimates they'll number about 770,000. That's about a third of the 2.15 million federal workers, excluding postal employees, reported by the Bureau of Labor Statistics.

Some effects the shutdown will — or won't — have are becoming clear.

• In-process applications for mortgages backed by the Federal Housing Administration will not be interrupted, said George Gonzalez, a spokesman for the Department of Housing and Urban Development. Federally run mortgage insurers Fannie Mae and Freddie Mac will also be open.

• Health insurers and health care providers initially won't feel much impact, Moody's says, because federal payments under entitlement programs such as Medicare and Medicaid are considered essential government operations and not subject to the partial closing. Furloughs of administrative employees will eventually mean payment-processing delays if the government stays half-closed, analyst Steve Zaharuk said.

The biggest short-term impact will be on defense contractors, Herskovics said. They are healthier than in 2011, when Congress flirted with refusing to raise the debt ceiling. But even big contractors like Lockheed Martin, which said Monday it will stay open, are vulnerable to an extended shutdown because of relatively high debt and heavy reliance on federal contracts, he said.

Some contractors are not yet able to give a clear picture to the public of where they stand, said James Fisher, a spokesman at Booz Allen Hamilton, an information-technology consulting firm that gets 98% of its revenue from government clients.

"We're spending most of our time now gathering information and answering questions internally,'' Fisher said. "There are dozens of 'one-off' questions internally that have to be researched and answered so we can provide proper guidance to our employees.''